Interventions can take a variety of forms but often include economic incentives.
A baseline is a prediction of the quantified amount of an input to or output from an activity resulting from the expected future behavior of the actors proposing, and affected by, the proposed activity in the absence of one or more policy interventions, holding all other factors constant (ceteris paribus).
Additionality measures the net result, taking account of deadweight, leakage, displacement, substitution and economic multipliers.
Additionality becomes problematic when the parties claim that their behavior is being changed due to recognized intervention (e.g., because of the economic incentive provided by earning carbon offset credits), when in fact the intervention is having no effect on their behavior because other factors are dominant (e.g., earning a profit from an activity even without carbon credits).
The proposed project is therefore not truly additional, since it would have been implemented without the intervention (e.g., in the form of the carbon credit price signal).